This invention relates to lamps and particularly to arc discharge lamps. Still more particularly, the invention relates to arc discharge lamps employing a ceramic arc tube, a shield and mounting means for mounting said arc tube within the shield.
Metal halide arc discharge lamps are frequently employed in commercial usage because of their high luminous efficacy and long life. A typical metal halide arc discharge lamp includes a quartz or fused silica arc tube that is hermetically sealed within a borosilicate glass outer envelope. Recent advances in the art have employed a ceramic arc tube constructed, for example, from polycrystalline alumina. It is with the latter type that this invention is particularly concerned. The arc tube, itself hermetically sealed, has tungsten electrodes sealed into opposite ends and contains a fill material that may include mercury, metal halide additives and a rare gas to facilitate starting. In some cases, particularly in high wattage lamps, the outer envelope is filled with nitrogen or another inert gas at less than atmospheric pressure. In other cases, particularly in low wattage lamps, the outer envelope is evacuated.
It has been found desirable to provide metal halide arc discharge lamps with a shroud that comprises a generally tubular, light-transmissive member, such as quartz, that is able to withstand high operating temperatures. The arc tube and the shroud are coaxially mounted within the lamp envelope with the arc tube located within the shroud. Preferably, the shroud is a tube that is open at both ends.
In those lamps using an arc tube made from quartz or fused silica or like material, the arc tube has a generally tubular body sealed at the ends by a pinch seal. The pinch seals provide a flattened area on the arc tube that lends itself to receiving a mounting structure that both positions the arc tube within the shroud or shield and allows the entire structure to be mounted upon a suitable frame within an envelope.
The shroud or shield has several beneficial effects on lamp operation. In lamps with a gas-filled outer envelope, the shroud reduce convective heat losses from the arc tube and thereby improves the luminous output and the color temperature of the lamp. In lamps with an evacuated outer envelope, the shroud helps to equalize the temperature of the arc tube. In addition, the shroud effectively reduces sodium losses and improves the maintenance of phosphor efficiency in metal halide lamps having a phosphor coating on the inside surface of the outer envelope. Finally, the shroud improves the safety of the lamp by acting as a containment device in the event that the arc tube shatters.
In lamps using ceramic arc tubes, mounting the arc tube within a shroud has proven difficult and expensive. The ceramic arc tube has a tubular, often bulbous body with ceramic, cylindrical capillaries extending therefrom. The capillaries are relatively small, often having diameters of 3 mm or so, and contain the electrodes.
It is, therefore, an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to enhance the mounting of arc tubes within shrouds.
It is yet another object of the invention to achieve these objects in an inexpensive manner.
These objects are accomplished, in one aspect of the invention, by the provision of a spring clip that comprises a base that extends in a first plane and has an aperture centrally located therein. Upstanding walls, one at each end of the base, are provided. A first lip extends orthogonally away from one of the walls in a second plane and a second lip extends orthogonally away from the other wall in the second plane. The second plane is substantially parallel to the first plane. Flags formed respectively with the lips extend away from the lips in a plane transverse to the first and second planes.
The clips are used to form an assembly for a lamp. The assembly comprises a light source that has a center with projecting, opposite ends arrayed along a longitudinal axis. The ends are cylindrical in cross-section.
A tubular shroud surrounds the light source and is coaxial with the longitudinal axis. The shroud has two ends.
A pair of spring clips is provided, one at each end of the shroud. Each of the spring clips comprises a base in a first plane and has an aperture centrally located therein. Each aperture of one of the spring clips frictionally engages one of the cylindrical ends of the light source. Upstanding walls are provided, one at each end of the base of the clips, the walls lying adjacent to the interior surface of the shroud and at least a part of each of the walls frictionally engages the interior surface of the shroud.
A first lip extends orthogonally away from a first of the walls in a second plane and a second lip extends orthogonally away from the other of the walls, also in the second plane. The second plane is substantially parallel to the first plane. The lips of each of the clips engage an end of the shroud. Flags formed with the lips extend away from the lips in a plane transverse to the first and second planes.
When the assembly is put into a lamp at least some of the flags are affixed to a frame.
The clips thus provided are economical to manufacture and easy to use. The clips are elastic and, in one embodiment, can be attached to both ends of the arc tube and compressed to allow one of the clips to be passed into the shroud where it elastically unfolds at the required insertion distance.